The neutron detector is an elemental technology that supports the technology utilizing neutrons. Neutron detectors with higher performance are demanded, with the development of the technology utilizing neutrons in such fields as the medical field including boron neutron capture therapy, the academic research field including structural analysis by neutron diffraction, the non-destructive inspection field, and the security field including cargo inspection.
An important characteristic demanded of the neutron detectors is precision in counting neutrons (hereinafter “neutron counting precision”). The neutron counting precision is affected by various factors such as detection efficiency for neutrons and discrimination between neutrons and γ rays (which is hereinafter referred to as “n/γ discrimination”). The detection efficiency refers to a ratio of the number of neutrons counted by the detector, with respect to the number of neutrons incident upon the detector. If the detection efficiency is low, the absolute number of neutrons counted is small, therefore resulting in degradation of measurement precision. In addition, γ rays not only exist in natural radiation, but also are generated when neutrons hit a component of a neutron detecting system or hit an object to be inspected. Therefore, if the γ rays are mistakenly counted as the neutrons because of low n/γ discrimination, the neutron counting precision will degrade.
In general, a neutron capture reaction is utilized to detect neutrons since the neutrons have strong power to pass through a material without performing any interactions in the material. For example, a helium-3 detector has been known, which performs the detection by utilizing a proton and tritium generated by the neutron capture reaction between 3He and the neutron. This detector is a proportional counter filled with 3He gas, having high detection efficiency and being excellent in n/γ discrimination. However, there is a drawback that it is difficult to reduce the size of the detector, and therefore difficult to carry out measurement in a small area or a narrow space. Further, 3He is an expensive substance and is also limited in its amount.
Recently, a scintillation neutron detector having a neutron scintillator has been developed as an alternative to the helium-3 detector mentioned above. The neutron scintillator is a substance that emits light by interaction with neutrons incident thereon. Combining the neutron scintillator and a photodetector such as a photomultiplier tube can form the scintillation neutron detector. The above described various performances of the scintillation neutron detector utilizing the neutron scintillator depend on a constituent of the neutron scintillator. For example, if an isotope which exhibits high efficiency in the neutron capture reaction is contained in a high amount, detection efficiency for neutrons will improve. Examples of such an isotope are 6Li and 10B (see Patent Document 1).
In the scintillation neutron detector having the neutron scintillator, the photodetector detects light emitted from the neutron scintillator and outputs a pulse signal. In general, the number of neutrons is measured by strength of the pulse signal, so called a pulse height value. That is, with a predetermined threshold given for the pulse height value, an event showing a pulse height value exceeding the threshold is counted as incidence of neutrons, and an event showing a pulse height value not reaching the threshold is treated as noise.
In addition, a scintillation neutron detector having a neutron scintillator and a photodetector linked by an optical fiber (hereinafter referred to as an optical fiber-type scintillation neutron detector) has been developed as an application of the scintillation neutron detector having the neutron scintillator. In the optical fiber-type scintillation neutron detector, scintillation light generated by the neutron scintillator is transmitted by the optical fiber to the photodetector.
There has been an attempt to use a mixed powder of LiF containing 6Li and Ag-doped ZnS being a fluorescent material (hereinafter referred to as LiF/ZnS), or a glass scintillator containing 6Li (hereinafter referred to as a Li glass), as the neutron scintillator in the optical fiber-type scintillation neutron detector (see Non-Patent Documents 1 and 2).
Since the optical fiber-type scintillation neutron detector described above is suitable for measuring neutrons in a small area or a narrow space, it is utilized as a neutron monitor for boron neutron capture therapy (BNCT) or as a neutron monitor in a nuclear reactor.